The present invention relates generally to integrated circuit (IC) fabrication processes and, more particularly, to methods of forming high dielectric constant materials on silicon.
Current Si VLSI technology uses SiO2 as the gate dielectric in MOS devices. As device dimensions continue to scale down, the thickness of the SiO2 layer must also decrease to maintain the same capacitance between the gate and channel regions. Thicknesses of less than 2 nanometers (nm) are expected in the future. However, the occurrence of high tunneling current through such thin layers of SiO2 requires that alternate materials be considered. Materials with high dielectric constants would permit gate dielectric layers to be made thicker, and so alleviate the tunneling current problem. These so-called high-k dielectric films are defined herein as having a high dielectric constant relative to silicon dioxide. Typically, silicon dioxide has a dielectric constant of approximately 4, while it would be desirable to use a gate dielectric material with a dielectric constant of greater than approximately 10.
Because of high direct tunneling currents, SiO2 films thinner than 1.5 nm generally cannot be used as the gate dielectric in CMOS devices. There are currently intense efforts in the search for the replacement of SiO2, with TiO2 and Ta2O5 attracting the greatest attention. However, high temperature post deposition annealing, and the formation of an interfacial SiO2 layer, make achieving equivalent SiO2 thicknesses, also known as equivalent oxide thickness (EOT), of less than 1.5 nm very difficult. An EOT of about 1.0 nm, and below, is expected to be used for the 0.07 micrometer device generation.
Materials such as hafnium oxide (HfO2) and zirconium oxide (ZrO2) are leading candidates for high-k dielectric materials. The dielectric constant of these materials is about 20 to 25, which is a factor of 5-6 times that of silicon dioxide, meaning that a thickness of about 5-6 nm of these materials could be used to achieve an EOT of about 1.0 nm, assuming that the entire film is essentially composed of the high-k material. One problem with using high-k materials is that an interfacial layer of silicon dioxide, or a silicate layer, with a lower dielectric constant forms during standard processing.
Deposition of ZrO2, or HfO2, using atomic layer deposition (ALD) and tetrachloride precursors has been reported. Substrates heated to between 300xc2x0 C. and 400xc2x0 C. have been exposed to ZrCl4, or HfCl4, precursors alternating with water vapor in an attempt to form ZrO2 or HfO2 films respectively. However, it is difficult to initiate deposition on hydrogen terminated silicon surfaces. Hydrogen terminated silicon surfaces result from standard industry cleaning processes. These standard cleaning processes, which are often referred to as HF last clean, typically end in a quick dip of HF. This produces surfaces which are hydrogen terminated, also known as hydrogen passivated. With sufficient exposure of the silicon surface to the reactants, the deposition may eventually be initiated. But, this results in films that are rough with poor uniformity. Another problem with tetrachloride precursors is the incorporation of residual chlorine in the film. The chlorine impurities can result in long term reliability and performance issues.
Other precursors use Hf or Zr metal combined with organic ligands such as iso-propoxide, TMHD (2,2,6,6-tetrmethyl-3,5-heptanedionate), or combinations of organic ligands with chlorine. These precursors also have a problem initiating the film deposition on hydrogen terminated silicon surfaces and will incorporate carbon residues in the film. Large ligands may also take up enough space that steric hindrance will prevent the deposition of a uniform monolayer. Up until now, the successful implementation of ALD Zr and Hf oxides have been either on an initial layer of silicon oxide, silicon oxynitride, or in the form of a reduced dielectric constant silicate film, such as ZrSiO4 or HfSiO4. These initial layers may contribute significantly to the overall EOT.
Accordingly, a method of forming high dielectric constant materials, ZrO2 or HfO2, is provided. The methods are well suited to forming high dielectric constant materials on hydrogen terminated silicon surfaces, however the methods can be also used to form these materials on a variety of substrates.
A method is provided for forming zirconium oxide on a substrate comprises providing a semiconductor substrate within an atomic layer deposition chamber. Heating the substrate to a temperature within the atomic layer deposition regime. Introducing anhydrous zirconium nitrate into the chamber. Purging the chamber with nitrogen. And, introducing water vapor into the chamber, whereby a monolayer of zirconium oxide is deposited. The steps of introducing of anhydrous zirconium nitrate, purging the chamber with nitrogen, and introducing water vapor may each be repeated as necessary to produce a zirconium oxide film of the desired thickness.
A method is provided for forming hafnium oxide on a substrate comprises providing a semiconductor substrate within an atomic layer deposition chamber. Heating the substrate to a temperature within the atomic layer deposition regime. Introducing anhydrous hafnium nitrate into the chamber. Purging the chamber with nitrogen. And, introducing water vapor into the chamber, whereby a monolayer of hafnium oxide is deposited. The steps of introducing of anhydrous hafnium nitrate, purging the chamber with nitrogen, and introducing water vapor may each be repeated as necessary to produce a hafnium oxide film of the desired thickness.
A method is provided for forming a nanolaminate, which comprises hafnium oxide and zirconium oxide. The method comprises repeating the steps mentioned above with regard to forming zirconium oxide, and repeating the steps mentioned above with regard to forming hafnium oxide, and alternating these steps as desired to produce a nanolaminate, such as HfO2/ZrO2/HfO2/ZrO2.